How do you make a picture?

Transcription

1 Take-Away Messages LBSC 690 Session #11 Multimedia Human senses are gullible Images, video, and audio are all about trickery Compression: storing a lot of information in a little space So that it fits on your hard drive So that you can send it quickly across the network Jimmy Lin Wednesday, November 12, 2008 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States See for details How do you make a picture? Georges Seurat - A Sunday Afternoon on the Island of La Grande Jatte 1

2 What s a pixel? What s resolution? How do you get color? #99FF66 #9999FF 8 bits 8 bits 8 bits How do LCDs work? 2

3 How do digital cameras work? 2,048 x 1,536 = 3,145,728 3 MP 2,560 x 1,920 = 4,915,200 5 MP 3,264 x 2,448 = 7,990,272 8MP 3,648 x 2,736 = 9,980, MP Is a picture really worth 1000 words? (consider an image with 1024 x 768 resolution) Compression Goal: represent the same information using fewer bits Two basic types of data compression: Lossless: can reconstruct exactly Lossy: can t reconstruct, but looks the same Two basic strategies: Reduce redundancy Throw away stuff that doesn t matter Run-Length Encoding Large regions of a single color are common Record # of consecutive pixels for each color An example with text: Sheep go baaaaaaaaaa and cows go moooooooooo Sheep go ba<10> and cows go mo<10> Using Dictionaries Data often has shared substructure, e.g., patterns Create a dictionary of commonly seen patterns Replace patterns with shorthand code An example with text: t The rain in Spain falls mainly in the plain The r* ^ Sp* falls m*ly ^ the pl* (*=ain,^=in) 3

4 Palette Selection No picture uses all 16 million colors Select a palette of 256 colors Indicate which palette entry to use for each pixel Look up each color in the palette What happens if there are more than 256 colors? Discrete Cosine Transform Images can be approximated by a series of patterns Complex patterns require more information than simple patterns Break an image into little blocks (8 x 8) Represent each block in terms of basis images This is GIF! Full quality (Q = 100): 83,261 bytes Medium quality (Q = 25): 9,553 bytes This is JPEG! Average quality (Q = 50): 15,138 bytes Low quality (Q = 10): 4,787 btes When should you use jpegs? When should you use gifs? 4

5 Raster vs. Vector Graphics Demo! Raster images = bitmaps Actually describe the contents of the image Vector images = composed of mathematical curves Describe how to draw the image What happens when you scale vector images? What happens when you scale raster images? Basic Video Coding How do you make video? Display a sequence of images Fast enough to trick your eyes (At least 30 frames per second) NTSC Video 60 interlaced half-frames/sec, 720x486 HDTV 30 progressive full-frames/sec, 1280x720 5

6 Video Example Typical low-quality video: 640 x 480 pixel image 3 bytes per pixel (red, green, blue) 30 frames per second Storage requirements: 26.4 MB/second! A CD-ROM would hold 25 seconds 30 minutes would require 46.3 GB Some form of compression required! Video Compression One frame looks very much like the next Record only the pixels that change Frame Reconstruction I 1 I 1 +P 1 I 1 +P 1 +P 2 I 2 updates I frames provide complete image What is sound? How does hearing work? How does a speaker work? How does a microphone work? P frames provide series of updates to most recent I frame P 1 P 2 Basic Audio Coding Sample at twice the highest frequency 8 bits or 16 bits per sample Sampler How do MP3s work? The human ear cannot hear all frequencies at once, all the time Don t represent things that the human ear cannot hear Speech (0-4 khz) requires 8 KB/s Standard telephone channel (8-bit samples) Music (0-22 khz) requires 172 KB/s Standard for CD-quality audio (16 bit samples) 6

7 Human Hearing Response Frequency Masking Experiment: Put a person in a quiet room. Raise level of 1kHz tone until just barely audible. Vary the frequency and plot the results. Experiment: Play 1kHz tone (masking tone) at fixed level (60 db). Play test tone at a different level and raise level until just distinguishable. Vary the frequency of the test tone and plot the threshold when it becomes audible. Temporal Masking If we hear a loud sound, then it stops, it takes a while until we can hear a soft tone at about the same frequency. MP3s: Psychoacoustic compression Eliminate sounds below threshold of hearing Eliminate sounds that are frequency masked Eliminate sounds that are temporally masked Eliminate stereo information for low frequencies Streaming Audio and Video How do you deliver continuous data over packet-switched networks? Simultaneously: Receive downloaded content in buffer Play current content of buffer Analogy: filling and draining a basin concurrently Internet Media Sever Buffer 7

8 Example: Internet Telephony to buffer or not to buffer Internet radio YouTube Skype Instant Messenger IP Phones: Network Issues Network loss: packets lost due to network congestion Delay loss: packets arrives too late for playout at receiver Loss tolerance: depending on voice encoding packet loss rates between 1% and 10% can be tolerated IP Phones: Playout Delay Receiver attempts to playout each chunk exactly q ms after chunk was generated Chunk has time stamp t: play out chunk at t+q Chunk arrives after t+q: data arrives too late for playout, data lost Tradeoff for q: Large q: less packet loss Small q: better interactive experience Take-Away Messages Human senses are gullible Images, video, and audio are all about trickery Compression: storing a lot of information in a little space So that it fits on your hard drive So that you can send it quickly across the network 8